Aluminum substrates have been widely utilized as substrates for a magnetic recording medium such as a magnetic disk or the like as an information recording medium, but along with high density recording together with downsizing of the magnetic disk, the aluminum substrates tend to be replaced by glass substrates each exhibiting excellent strength and excellent recording surface flatness, compared with the aluminum substrates.
In recent years, reduction of flying height of a magnetic head (glide•height) has been demanded in order to improve the recording density, and to respond to this demand, the surface of a magnetic recording medium, that is, the surface of a glass substrate for a magnetic recording medium is desired to be finished as an extremely uniform surface.
In Patent Document 1, disclosed is a magnetic disk with a fine undulation having a period of 0.1-5 mm, and an amplitude of 0.1-1 nm.
In Patent Document 2, disclosed is a magnetic disk for which the undulation on the surface when measuring at a measurement wavelength of 5.0 mm has an amplitude Wa of 0.1-0.5 nm; the fine undulation formed on the foregoing undulation when measuring at a measurement wavelength of 30-200 μm has a mean amplitude Wb of 0.3 nm or less; and a value Wb/Wa obtained by dividing mean amplitude of the fine undulation Wb with amplitude of the undulation Wa is 0.6 or more.
Flying height of a magnetic head is often indicated by a Glide Avalanche value (GA value) via evaluation of the flying height of the magnetic head (for example, Patent Document 3). A magnetic recording medium becomes suitable for high recording density when the GA value becomes smaller.